The game needs to open up

[Alleluia]

How hard would it be to try out an expansion where zones aren't locked by MSQ,

Honestly, am not opposed to this bit. I would love to see some expac content dive back in to old zones. Especially the ARR zones, which feel the most lived in in the game. Bonus points if we get origin-specific content again. I just talked to all the NPCs in the little instanced Jeuno for the new 24 man and one of the NPCs asked where you are from. Having my starter city mentioned was unexpectedly nice.

-where quests don't have braindead markers telling you where to go

The option to just turn them off could suffice here, but it would require writing to pick up the slack and give more direction more consistently.

-where dungeons are nothing like the stale formula we have

Dungeons are fine and have only gotten more interesting, mechanics-wise. Though if I never see another spinning pointer hand mechanic, it'll be too soon.

-where underwater isn't just a pretty landscape and combat/breathing mechanics are tried out.

I have never seen a game where underwater combat went well. Its like mounted combat. Its epic if it's in a cinematic, fun if its a bit on rails in a set piece; but just freeform? No. Even GW2 underwater combat is meh and its probably one of the best mmo examples. Maybe, maybe, add creatures you need to avoid in some places with like an OGCD skill that lets you stun them for time to get away if you end up too near them or something.

-Where exploration and open world events are tried through the entire introduced zones instead of designated exploration instances like Eureka and Bozja.

How is this different than fates->special fates?

-Where some side quests are just damn hard

I'm for innovation in sidequesting. Though a solo sidequest leading to an overworld miniboss is less than ideal. We had that with relic steps in ARR requiring fates. Its not fun. However, if by "hard" you mean "not a readily obvious solution" or "requires a lot of steps", I'm one of those people who won't google it and will find it fun.

-Where some open world dungeons are introduced, and the mobs are just lethal

Aren't these exploration zones?